This invention relates generally to signal detection in systems utilizing arrayed receptors for acoustic and electromagnetic wave signals as in sonar, radar, communications and seismic wave detection systems. More particularly, the invention relates to the processing of signals as received by such receptor arrays to accomplish array beamforming and to extract useful signal output from signals received in company with noise and interference.
Many reports have appeared in recent literature on efforts to achieve optimal space-time processing of signals in array systems particularly for sonar application, so as to maximize system capabilities to detect useful signals immersed in noise. From these efforts there has evolved an optimal processor concept which is fairly well defined, and which also is narrowly defined in the sense that there is a strong similarity among most if not all of the so-called optimal processors. Generally such processors are composed of a beamformer or spatial processor followed by a filter, and for plane wave signals the beamformer is common to all and only the filter reflects the particular criterion of optimality selected. In all cases the spatial processor or beamformer functions to maximize the detectability of deterministic known signals immersed in gaussion interference, and it accomplishes this through a set of filter functions which achieve a maximized signal-to-interference power density ratio at each frequency. In this sense the beamformer may properly be considered a spatial prewhitener; at each frequency it suppresses peaks in the angular power density function of the noise or interference.
While optimal processors thus display a commonality of concept and basic function, attempts at their implementation have employed a variety of different approaches and experienced varying degrees of success in achieving optimal processing in practical systems. Many practical systems, for example, employ amplitude and phase steering, and arrays thus steered normally are not capable of generating an independent radiation pattern at each frequency as required for the theoretically optimal processor. Another problem arises where the characteristics of the useful signal and the noise do not enable temporal discrimination between them; in such cases it is difficult to accomplish the desired prewhitening without suppression of useful signals along with the noise.